1. Field of the Invention
The present invention relates to a light modulation element and a light modulation apparatus having such an element and, more particularly, to a light modulation element for modulating light in accordance with an input signal and for use in a printer or a display, and a light modulation apparatus using such an element.
2. Description of the Prior Art
Light modulation elements of the type described above include a cantilever element utilizing electrostatic attraction force as described in "The Mirror-Matrix Tube: Novel Light Valve for Projection Displays: (IEEE TRANS. ELECTRON DEVICES, Vol, ED-22, No. 9, 1975), "Micromechanical light modulator array fabricated on silicon" (Appl. Phys. Lett., Vol. 31, No. 8 1977), or Japanese Laid-open Patent Application No. 17525/1984 of the same applicant as that of the present invention; a light modulation element utilizing the thermo-optical effect as described in Japanese Patent Application Laid-open No. 68723/1984; and a light modulation element or liquid crystal element utilizing a crystal having an electro-optical effect as described in Japanese Patent Application Laid-open No. 5526/1981.
FIG. 1 shows an example of an arrangement of an arrayed modulation element using a plurality of cantilever mirrors. Referring to FIG. 1, modulation portions 1a, 1b, 1c and 1d are reflection surfaces which are separated from each other by cut portions 3a, 3b, 3c and 3d leaving connecting portions 2a, 2b, 2c and 2d. The deflecting directions of the reflecting surfaces of the modulation portions 1a, 1b, 1c and 1d about the connecting portions 2a, 2b, 2c and 2d can be independently controlled by input signals, and can thus deflect light incident on the example.
FIG. 2 shows an example of a light apparatus using such a light modulation element 5. Light from an illumination system 6 illuminates the element 5. When the modulation portions 1a, 1b and 1c are not in operation, the light reflected thereby forms a light source image 7 in an arbitrary space position. However, when the modulation portions 1a, 1b and 1c are in operation, light incident on the modulation portions 1a, 1b and 1c is reflected in a direction which is different from that in the case wherein the elements 1a, 1b and 1c are not in operation. An imaging system 8 is arranged near the light source image 7 such that the light source image 7 does not enter the entrance pupil. An observation surface 9 is at a conjugate position with the element 5 with respect to the imaging system 8. The light reflected by the elements 1a, 1b and 1c is projected by the imaging system 8 onto the observation surface 9 as spots 10a, 10b and 10c. The observation surface 9 is a photosensitive body in the case of a printer, and is a screen in the case of a display. However, since the cut portions 3b and 3c as non-modulation portions are between the respective modulation elements 1a, 1b and 1c, images 11b and 11c corresponding to these portions 3b and 3c are formed on the observation surface 9. When the light modulation apparatus as shown in FIG. 2 using such a light modulation element is used in an optical apparatus such as a printer, a uniform exposure distribution along the array direction of the modulation portions cannot be obtained due to the presence of the cut portions. This problem is not limited to the light modulation element as shown in FIG. 1, and the same problem recurs in other known light modulation elements of similar type. FIG. 3 shows a method normally adopted to resolve this problem.
A light modulation element shown in FIG. 3 has light modulation portions 21, 22, 23, 24 and 25, non-modulation portions 31, 32, 33, 34 and 35, and cut portions S1, S2, S3, S4 and S5. As shown in FIG. 3, the modulation portions are staggered such that the odd-numbered modulation portions 21, 23, 25, . . . are arranged on an array AA' and the even-numbered modulation portions 22, 24, . . . are arranged on an array BB'. Data signals corresponding to one line are divided into the odd- and even-numbered arrays separated by a predetermined time period and are controlled to be in a single array on the final observation surface.
However, the conventional method shown in FIG. 3 is still subject to the following problems. First, the ratio of the amount of light which is deflected to the total amount of illuminating light incident on the elements, i.e., the utilization effect is low. The utilization efficiency is at or less than 50% since the total area of the nonmodulation portions 31, 32, . . . , 35 is larger than that of the modulation portions 21, 22, . . . , 25. Second, if the shape of each modulation portion is assumed to be a square having one side x, a distance l between the arrays AA' and BB' must be larger than the side x. However, in order to allow processing of input signals supplied sequentially, a given line must be modulated by the array AA', and input signals supplied in a time interval up to modulation by the next array BB' must be temporarily stored in a buffer memory. The larger the distance l, the larger the number of buffer memories required for this purpose. In view of this, in order to reduce the load of the electrical system, it is preferable that the distance l be decreased to a minimum.
FIGS. 4A, 4B and 4C show other conventional examples of light modulation apparatus using small reflecting surfaces exemplified by the cantilever mirror. FIG. 4A is a sectional view of the small reflecting surfaces. FIG. 4B is a plan view of the small reflecting surfaces. FIG. 4C is a diagram showing a display system using such a light modulation element. Small reflecting surfaces 1a, 1b and 1c have square shapes each having a side of 50 .mu.m. The reflecting surfaces 1a, 1b and 1c are arranged in an array as separated by air gaps 2 of several microns. Each small reflecting surface is divided by cut portions 7a and 7b into four reflecting portions which are formed integrally with each other through an intersecting portion 3. Silicon columns 2a, 2b and 2c support the respective small reflecting surfaces at the intersecting portions 3. The columns 2a, 2b and 2c are fixed on a substrate S. Regions 5a capable of storing charges are formed in the surface portions of the substrate S which oppose the small reflecting surfaces. The regions 5a are isolated by insulating layers 4. 7a and 7b represent cut portions of the small reflecting surface. When a charge is stored in the region 5a, the reflecting portions of the corresponding small reflecting surface 1 are deformed about the intersection 3 by the electrostatic attraction force. This deformation in the reflecting portion changes the reflecting direction of incident light, thereby deflecting the incident light in a direction. Static changes are selectively injected into or extracted from the respective portions 5a, so that the reflecting direction of light at each reflecting surface is controlled and the incident light is modulated thereby.
In an apparatus as shown in FIG. 4C using such a light modulation element, a lens 12 images light from a light source 11 onto a stopper 13 having a reflecting surface. An objective lens 14 has its focal point aligned on the surface of the stopper 13. The small reflecting surfaces 1a, 1b and 1c are arranged at the opposite focal point side of the objective lens 14. An imaging lens 15 and a screen 16 are arranged behind the stopper 13. Light from the light source 11 is collimated into parallel light by the objective lens 14 through the lens 12 and the stopper 13 and becomes incident on the small reflecting surfaces. If the small reflecting surfaces are not deformed, the light is returned to the light source through the incident light path in the reverse order. However, if the small reflecting surface 1b is deformed, the light is deflected, and the light is collimated into parallel light by the objective lens 14, thereafter the light is partially shielded by the stopper 13 but is mostly focused by the imaging lens 15 and reaches the screen 16. In this light modulation element, since incident light is diffracted in the gaps (2, 7a, and 7b) between the reflecting surfaces, the diffracted light generates noise light. The diffracted light is mostly reflected in two orthogonal directions in accordance with the shape of the gaps (2, 7a, and 7b). In this element, the diffracted light is removed by forming the shape of the stopper 13 at the spectrum plane into a cross shape. This optical system is a Schlieren optical system which is conventionally known. Since such a Schlieren optical system is subject to incidence of non-modulated light having a high energy density, countermeasures against heat and surface reflection must be taken. In addition, the size, position, rotation or the like of the stopper 13 must be controlled with high precision. When an array of a plurality of imaging elements is used, in order to obtain a compact optical system the illumination system and the Schlieren system are rendered complex in structure and require high precision of arrangement.
As in the light modulation element described above, light modulation elements for modulating incident light by changing its phase are also subject to the same problem of diffracted light depending upon the element structure. Such light elements are described, e.g., in Japanese Patent Application Laid-open No. 5525/1981 and No. 68723/1984.
FIG. 5 shows a conventional light modulation apparatus as an example of a transmission-type light modulation element. Light 17 incident on a dielectric crystal 19 is output as non-deflected light 18a. However, when a heater 20 is energized, the incident light 17 is converted into deflected light 18b by a refractive index distribution 20' formed in the crystal.
In such a light modulation element, the separation angle of modulated light and non-modulated light is as small as several degrees. Therefore, when this element is used for a light modulation apparatus by shielding the modulated or the non-modulated light, such light shielding must be performed at a position sufficiently separated from the light modulation element in order to provide a good S/N ratio. When a plurality of optical modulation elements as shown in FIGS. 1 to 5 are arranged in a one-dimensional array or a two-dimensional array, non-modulated light transmitted through or reflected by non-modulation regions between modulation regions must be sufficiently separated from modulated light.